Different arrangements are possible. For example, figure 1
shows a split range pressure controller on a separator with two valves, one to
the flare and one to the compressor suction. In this case, the ‘split’ is
configured as follows:

Controller output 0%
Both valves are closed.

Controller output 25%
Valve A is 50% open and Valve B still closed.

Controller output 50%
Valve A is fully open and Valve B closed.

Controller output
75% Valve A is fully open and Valve B 50% open.

Controller output
100% Both valves are fully open.

The idea is that the suction valve is used for normal
pressure control while the flare valve only opens to disperse high pressures.

Figure 1.
Split range control scheme

In this application, the flare valve will need to open
quickly in response to high pressures, but the compressor suction valve will
need to move much more slowly to prevent instability in the compressors. The
main problem with split range control is that the controller only has one set of
tuning parameters. If the controller is tuned to be fast acting to optimise the
performance of the flare valve, the suction valve will also move rapidly to
produce unstable gas flows to the compressors. If the controller is tuned slower
to stabilise the compressors, then the flare valve will not open fast enough as
the pressure rises. A further issue is that the process response of the route to
flare generally differs to the process response of the route to the compressors,
so both routes will anyway require very different tuning for optimal control.

The solution is to replace the split range controller with
two independent controllers, both reading the same pressure transmitter, but one
controlling the flare valve and the other the suction valve. Not only can each
controller be tuned correctly for its dedicated service, but different setpoints
can also be used to prevent the flare valve from ‘popping’ unnecessarily.

Figure 2.
Correct implementation

The ease with which a split range controller can be
replaced with two ordinary controllers depends on a number of factors. If the
‘split’ is calculated in the DCS or PLC so that each valve has its own output
from the control system, then the addition of a new controller is simply a
matter of software configuration. However, occasionally, the control system only
has one output wired to both controllers and the ‘split’ produced by
configuration of the valve positioner. A new output will then be required from
the control system to one of the valves and the valve positioners must be
reconfigured to operate over the full 0-100% output range.